The strength of glass depends on the intactness of the glass surface. Immediately after it is produced, the strength of glass is highest. However, the strength falls off when the glass surfaces have nicks or scratches. Numerous methods are therefore known for protecting the surfaces of objects of glass against mechanical damage.
For example, it is known to treat glass objects immediately after they are shaped at temperatures between about 370.degree. and 750.degree. C. with organic and/or organic compounds, especially of titanium, tin or zirconium (the so-called hot-end finishing). With this treatment, thin, colorless, transparent metal oxide coatings are formed on the surfaces of the hot-finished glass objects.
To further increase the scratch hardness and the strength of glass objects and to improve their sliding ability, it is known to apply an additional organic coating applied on glass objects finished with metal oxides in a so-called cold-end finishing process.
For example, a method is described in the German Pat. No. 1,291,448 to increase the scratch hardness and strength of glass objects, especially glass bottles, by producing a thin, colorless, transparent coating on the exterior surfaces of the glass objects. The characteristic feature of this method consists therein that thin layers of a pyrolyzable inorganic salt or of a pyrolyzable organic compound of titanium, zirconium, tin or vanadium, applied on the glass objects, are decomposed pyrolytically on the glass objects at temperatures between 370.degree. and 705.degree. C. to the corresponding metal oxides, the glass objects are cooled to temperatures between 230.degree. and 65.5.degree. C. and an olefin polymer, a polyurethane, a polystyrene or an acetate salt of an alkylamine is sprayed on the still hot glass surfaces. An example of an olefin polymer is a polyethylene wax of low molecular weight, which is used in the form of an aqueous emulsion. As emulsifier, the alkali metal salt of a fatty acid, especially potassium oleate is used. The thickness of the coating on the glass object is about 1 .mu.m. These films of polyethylene wax are, however, not resistant to soap suds and, when the vessel is cleaned, are detached after a relatively short time by the surface active substances of the cleaning liquor.
From the German Pat. No. 2,412,068, a glass container is known, which is coated with a metal oxide, especially with tin oxide, and, on top of this, with an insoluble organic compound, and which has an intermediate layer of a fatty acid between these two coatings. According to this patent, the glass container is finished by first coating it in a hot-end treatment with a metal oxide, especially with tin oxide, applying a fatty acid on the metal oxide coating while the container is at a temperature of 90.degree. to 130.degree. C. and then applying an insoluble organic coating, while the vessel is at a temperature of 90.degree. to 150.degree. C. As fatty acid, oleic acid may be used. The insoluble organic coating preferably consists of polyethylene. The alkali resistance is said to be increased by the intermediate layer formed from the fatty acid. The polyethylene itself is applied in the form of a polyethylene emulsion, which contains potassium oleate as emulsifier. The resistance of these insoluble organic coatings to attack by hot, alkaline washing liquors, however, is stil not satisfactory. The method, moreover, is technically cumbersome and disadvantageous due to the need for the application of three layers. The use of alkali salts of fatty acids as emulsifiers generally has the disadvantage that polyethylene dispersions are unstable in the presence of salts which cause water to be hard.